What's the weather for Mars and Venus? The Hubble telescope has given astronomers a peek. The telescope is serving as an interplanetary weather satellite for studying the climate on Earth's neighboring worlds, Mars and Venus.

To the surprise of researchers, Hubble is showing that the Martian climate has changed considerably since the unmanned Viking spacecraft visited the Red Planet in the mid-1970s. The Hubble pictures indicate that the planet is cooler, clearer, and drier than a couple of decades ago. In striking contrast, Hubble's observations of Venus show that the atmosphere continues to recover from an intense bout of sulfuric "acid rain," triggered by the suspected eruption of a volcano in the late 1970s.

"The weather on Mars: another cool and clear day. Low morning haze will give way to a mostly sunny afternoon with high clouds. The forecast for Venus: hot, overcast, sulfuric acid showers will continue. Air quality is slightly improved as smog levels subside."

NASA's Hubble Space Telescope is serving as an interplanetary weather satellite for studying the climate on Earth's neighboring worlds, Mars and Venus.

To the surprise of researchers, Hubble is showing that the Martian climate has changed considerably since the unmanned Viking spacecraft visited Mars in the mid-1970s, which was the last time astronomers got a close-up look at weather on the Red Planet for more than just a few months. Hubble images of fleecy clouds, and spectroscopic detection of an ozone abundance in Mars' atmosphere, all indicate that the planet is cooler, clearer and drier than a couple of decades ago.

In striking contrast, Hubble's spectroscopic observations of Venus show that the atmosphere continues to recover from an intense bout of sulfuric "acid rain" triggered by the suspected eruption of a volcano in the late 1970s. This is similar to what happens on Earth when sulfur dioxide emissions from coal power plants are broken apart in the atmosphere to make acid rain. On Venus, this effect takes place on a planetary scale.

Although the close-up visits by numerous unmanned spacecraft provided brief snapshot glimpses of weather on these planets, the long-term coverage offered by Hubble has never before been possible. Knowledge about the weather is critical to planning future missions to these worlds. In the case of Mars, being able to predict the weather will be critical prior to human exploration and, perhaps eventually, colonization.

Studying conditions on Mars and Venus might also lead to a better understanding of Earth's weather system. Apparently, processes that occurred early in the solar system's history sent terrestrial planets along very different evolutionary paths. The neighboring planets are grand natural laboratories for testing computer models that will lead to a general theory of the behavior of planetary atmospheres.

Four years, (or two Mars years') worth of Hubble observations show that the Red Planet's climate has changed since the mid-1970's. "The Hubble results show us that the Viking years are not the rule, and perhaps not typical. Our early assumptions about the Martian climate were wrong," said Philip James of the University of Toledo.

"There has been a global drop in temperature. The planet is cooler and the atmosphere clearer than seen before," said Steven Lee of the University of Colorado in Boulder. "This shows the need for continuous monitoring of Mars. Space probes provided a close-up look, but it's difficult to extrapolate to long-term conditions based upon these brief encounters."

The researchers attribute the cooling of the Martian atmosphere to diminished dust storm activity, which was rampant when a pair of NASA Viking orbiter and lander spacecraft arrived at Mars in 1976. Two major dust storms occurred during the first year of the Viking visits, which left fine dust particles suspended in the Martian atmosphere for longer than normal. Warmed by the Sun, these dust particles (some only a micron in diameter, about the size of smoke particles) are the primary source of heat in the Martian atmosphere.

"Hubble is showing that our early understanding based on these visits is wrong. We just happened to visit Mars when it was dusty, and now the dust has settled out," Lee said. "We are going to have to look at Mars for many years to truly understand the workings of the climate," said Todd Clancy, of the Space Science Institute, Boulder, Colorado.

Knowledge about the Martian climate has been limited by the fact that ground-based telescopes can only see weather details when Earth and Mars are closest – an event called opposition – that happens only once every two years. Though Hubble has observed Mars only for four years, the observations are equivalent to 15 years of ground- based observing because Hubble can follow seasonal changes through most of Mars' orbit.

Though the Mariner and Viking series of flyby, orbiter and lander spacecraft that visited Mars in the late 60's and 70's provided a close-up look at Martian weather, these were snapshots of the planet's complex climate. Hubble provides the advantage of a global view - much like the satellites that monitor Earth's weather, and can follow martian seasonal changes over many years. When Mars is closest to Earth, Hubble returns near-weather satellite resolution.

Mars: No Lack of Ozone

Although there has been concern about a lack of ozone (a form of molecular oxygen created by the effects of sunlight on an atmosphere), dubbed the "ozone hole" over Earth's poles, there are no ozone holes on Mars. By contrast, the planet has a surplus of ozone over its northern polar cap, as first identified by the Mariner 9 spacecraft in 1971. (However the Martian atmosphere is different enough from Earth's that few parallels can be drawn about processes controlling the production and destruction of ozone.) Hubble's ultraviolet sensitivity is ideal for monitoring ozone levels on a global scale. The Martian ozone is yet another indication the planet has grown drier, because the water in the atmosphere that normally destroys ozone has frozen-out to become ice-crystal clouds. Spectroscopic observations made with the Faint Object Spectrograph (FOS) show that ozone now extends down from Mars' north pole to mid and lower latitudes. However, the Martian atmosphere is so thin, even this added ozone would offer future human explorers little protection from the Sun's harmful ultraviolet rays.

Seasons on Mars

The fourth planet from the Sun, Mars is one of the most intensely scrutinized worlds because of its Earth-like characteristics. Mars is tilted on its axis by about the same amount Earth is, hence Mars goes through seasonal changes. However, because Mars' atmosphere is much thinner than Earth's, it is far more sensitive to minor changes in the amount of light and heat received from the Sun. This is intensified by Mars' orbit that is more elliptical than Earth's, so it's range of distance from the Sun is greater during the Martian year. Mars is now so distant, the sun is nearly 25% dimmer than average. This chills Mars' average temperature by 36 degrees Fahrenheit (20 degrees Kelvin). At these cold temperatures, water vapor at low altitudes freezes out to form ice-crystal clouds now seen in abundance by Hubble.

"Clouds weren't considered to be very important to the Martian climate during the Viking visits because they were so scarce," says Clancy. "Now we can see where they may play a role in transporting water between the north and south poles during the Martian year." Seasonal winds also play a major role is transporting dust across Mars' surface, and rapidly changing the appearance of a region. This gave early astronomers the misperception that Mars' shifting surface color was evidence of vegetation following a season cycle.

As clearly seen in the Hubble images, past dust storms in Mars' southern hemisphere have scoured the plains of fine light dust and transported the dust northward. This leaves behind a relatively coarser, less reflective sand in the southern hemisphere.

Venus: No Evidence for New Volcanic Eruptions

Hubble spectroscopic observations of Venus taken with the Goddard High Resolution Spectrograph provide a new opportunity to look for evidence of volcanic activity on the planet's surface. Though radar maps of the Venusian surface taken by the Magellan orbiter revealed numerous volcanoes, Magellan did not find clear cut evidence for active volcanoes.

Hubble can trace atmospheric changes that might be driven by volcanism. An abundance of sulfur dioxide in the atmosphere could be a tell-tale sign of an active volcanos. Sulfur dioxide was first detected by the Venus Pioneer probe in the late 1970s and has been declining ever since. The Hubble observations show that sulfur dioxide levels continue to decline. This means there is no evidence for the recurrence of large scale volcanic eruptions in the last few years.

Ejected high into Venus' murky atmosphere, this sulfur dioxide is broken apart by sunlight to make an acid rain of concentrated sulfuric acid. This is similar to what happens on Earth above coal-burning power plants - but on a much larger and more intense scale.

Future Plans

More Hubble observations of Mars and Venus are critical to planning visits by future space probes. In particular, both robotic and human missions to Mars will need to be targeted for times during the Martian year when there is a minimal chance of getting caught in a dust storm. Knowing whether the atmosphere is relatively hot or cold is crucial to planning aerobraking maneuvers, where spacecraft use the aerodynamic drag of an atmosphere to slow down and enter an orbit around the planet. This reduces the amount of propellant needed for the journey. "If the atmosphere is more extended than expected the added friction could burn up an aerobraking spacecraft, just as Earth's atmosphere incinerates infalling meteors," says James.

Ultimately, knowing the Martian climate will be an fundamental prerequisite for any future plans to establish a permanent human outpost on the Red Planet.